Light-emitting diode (LED) based illumination devices are increasingly used for a wide variety of lighting and signaling applications. LEDs offer advantages over traditional light sources, such as incandescent and fluorescent lamps, including long lifetime, high lumen efficacy, low operating voltage and fast modulation of lumen output.
Efficient high-power LEDs are often based on blue light emitting InGaN materials. To produce an LED based illumination device having a desired color (e.g., white) output, a suitable wavelength converting material, commonly known as a phosphor, may be used which converts part of the light emitted by the LED into light of longer wavelengths so as to produce a combination of light having desired spectral characteristics. The phosphor may for example be embedded in an organic encapsulant material, such as epoxy, applied on top of the LED, or it may be pre-formed into a ceramic self-supporting layer which may be applied on the LED. Advantageously, a ceramic phosphor layer is more robust and less temperature sensitive than conventional organic phosphor layers. Such a ceramic phosphor layer is attached to the LED by means of an optical bond. Typically, materials used for such bonds include optical silicones, which have high photothermal stability, required by the operating conditions of the LED, and high transparency.
However, the commonly used optical bonds have a relatively low refractive index, typically in the range of 1.4 to 1.58, compared to the refractive index of the top layer of the LED through which light is extracted (which may be a top contact, e.g. GaN (refractive index of about 2.42), or a sapphire (refractive index of about 1.77) growth substrate). As a result, the critical angle for total internal reflection of light incident on the bond from the LED is relatively small. Light incident on the bond at angles exceeding the critical angle is not directly extracted from the LED. Also, the low refractive index of the bond compared to the refractive index of the ceramic phosphor results in limited transmission due to reflections at the interface. Thus, conventional optical bonds result in limited light extraction and transmission.
WO2007/138502 discloses an inorganic phosphor body for an LED comprising an inorganic luminescent material. A bonding precursor material is arranged an a surface of the inorganic phosphor body and the bonding precursor material comprises an at least partly hydrolyzed organically modified silane. The bonding precursor may further comprise oxide, which serves to increase the bonds index of refraction, which in turn enhances the light coupling capability of the bond. The refractive index may also be tailored within limited amounts by choosing an appropriate ratio between methyl and phenyl-modified silanes in the reaction mixture.
However, in spite of the bonding precursor presented in WO2007/138502, there remains a need in the art for improved optical bonds.